The storage or guidance of ions in volumes of any form defined by real or virtual walls requires reflection of the ions at or near the walls without the ions being discharged. For example, mechanical enclosure is ineffective because the ions are discharged at the physical walls. Up until now, ion-conserving reflections have been limited to two-dimensional and three-dimensional radio frequency (RF) multipole fields. These are more general forms (with more poles) of the two-dimensional and three-dimensional RF quadrupole fields invented by Wolfgang Paul and Helmut Steinwedel. Multipole rod systems have been used for several years for the guidance of ions in bad or moderate vacuums where collisions with a residual gas damp the movement of the ions.
In multipole rod systems, two-dimensional multipole fields are spanned between at least two pairs of rods, arranged evenly on the surface of a cylinder parallel to its axis. The two phases of an RF voltage are fed to the rods, opposite polarities existing between neighboring rods. Two pairs of rods span a quadrupole field, increasing numbers of rod pairs span hexapole, octopole, decapole, and dodecapole fields. The fields are called two-dimensional because any cross-section perpendicular to the axis exhibits the same field distribution; there is no dependence of the field distribution on the relative location along the axis of the device.
Three-dimensional multipole fields form the class of RF multipole ion traps. They consist of at least one ring electrode (the number of ring electrodes depending on the type of trap) and exactly two end cap electrodes. One ring electrode and the obligatory end cap electrodes span a quadrupole ion trap, two rings plus end caps form a hexapole, three rings produce an octopole, and four rings a decapole ion trap.
Radio frequency multipole rod systems are frequently used either as mass filters for inexpensive mass spectrometers, or as ion guides for transporting ions between ion production and ion consumption devices, particularly in feeding mass spectrometers of any type. Radio frequency multipole rod systems are favorably suited as ion guides for ion trap mass spectrometers, such as RF quadrupole ion traps or ion cyclotron resonance (ICR) mass spectrometers. Ion trap mass spectrometers operate cyclically with ion filling phases and ion investigation phases, and ions must not be introduced during the investigation phases. Ions can be temporarily stored in such ion guides by reflecting end potentials (as described in U.S. Pat. No. 5,179,278). Temporary storage of ions produced during the ion investigation phase therefore allows an increase in the duty cycle of the ion source. Furthermore, such ion guides can be used to thermalize ions produced outside the vacuum system of a mass spectrometer, and accelerated by the process of introducing them into the vacuum system. Thermalization requires a collision gas, and the residual gas inside a differential pumping stage can easily be utilized as such (see, e.g., U.S. Pat. No. 4,963,736).
Multipole rod systems for the guidance of ions usually have small diameters to concentrate the ions in a narrow area around the axis. The narrow area forms a pointed virtual ion source for excellent optical focusing of the ions exiting the ion guide. The inner, open diameters of these rod systems amount frequently to 3 to 6 millimeters only, the rods are usually less than 1 millimeter in diameter, and the system is about 5 to 15 centimeters long. The rods are mounted to notches in ceramic rings. There are high requirements to the precision of the arrangement. The system is hard to produce and sensitive to vibrations and shock. The rods get bent very easily, and cannot be re-adjusted with the required precision.
It is the objective of this invention to create methods and devices for the reflection of charged particles at or above surfaces. It is further the object of the invention to enclose charged particles in arbitrarily formed volumes with or without openings, and to transport ions without losses. The invention should be suited to form narrow, long ion guides with a mechanically robust structure, having good aptitude for thermalization and temporary storage of ions. It should be possible to produce inexpensive mass filters by this invention.